1. Field of the Invention
The invention relates to a device for detecting an obstacle in the opening range of a movable closure element. The closure element may be for example, an electrically powered window pane or sunroof of a motor vehicle. The device is provided with a sensor electrode having a first end and a second end, a base electrode having a first end and a second end and being spaced away from the sensor electrode, and a control unit connected to the first end of the sensor electrode and the first end of the base electrode. The sensor electrode and the base electrode generate an electric field in the opening range of the closure element and are spaced away from each other at a constant distance in a first operational mode. The control unit detects in the first operational mode a change in capacitance caused by the presence of an obstacle in the electric field and provides a control signal for a drive powering the closure element.
2. Description of Related Art
Such a device represents an anti-trap guard serving to prevent part of the human body, for example, being trapped between the closure element and a frame surrounding the closure element at least in part. By the way they work, these known devices are distinguished as a direct working and an indirect working anti-trap guard, as described for example, in DE 199 06 562 A1. A direct working anti-trap guard is understood to be a device or system comprising sensors which directly detect an obstacle and communicate a corresponding signal to an electric motor powering for example, a window pane, unlike an indirect working anti-trap guard which detects the load acting on the electric motor and determines the presence of an obstacle by processing the specific parameters of the electric motor. For a direct working anti-trap guard a wide variety of sensors can be used. Depending on the nature of the sensor, known devices can be distinguished by anti-trap guards requiring physical contact with the obstacle and anti-trap guards sensing the proximity of the obstacle non-contactingly.
One anti-trap guard belonging to the first group is described in EP 0 638 701 B1. This tactile anti-trap guard comprises a weatherseal sealing a closure element and is provided with two electrically conductive portions each separate from the other. On physical contact with an obstacle in the opening range of the closure element the conductive portions are squeezed together resulting in a switching contact triggering an electrical control signal. A tactile anti-trap guard also reads from DE 199 13 105 A1.
Unlike the former, a proximity anti-trap guard is known from EP 0 648 628 B1. This known anti-trap guard comprises an extruded weatherseal in which an electrical conductor is integrated. The electrical conductor represents a sensor electrode which generates an electric field in the opening range of a window pane. Changes in the capacitance due to the presence of an obstacle in the electric field are detected by a control unit which provides a control signal for a motor powering the window pane. Should an obstacle nevertheless come into contact with the weatherseal, the motor is reversed as soon as the force exerted by the obstacle on the window pane exceeds a predefined maximum permissible value of, for example, 100 N. In this known anti-trap guard a change in the capacitance triggering a control signal for the motor also materializes when the position of a sensor electrode integrated in a movably mounted lip is altered by an obstacle.
In DE 43 20 548 C2 a device for the opening and closing of a side window of a motor vehicle is disclosed having a control unit which interrupts or reverses the drive of the side window as a result of a sensor signal. The sensor signal is obtained by the measuring of the capacitive resistance between two electrically conducting slot boundaries, for example by detuning an oscillating circuit or a Wheatstone resistance bridge. One of the slot boundaries is formed by a seal which seals the side window. The other slot boundary is either formed by an electrically conducting top edge of the side window or, if the conducting properties of the window are sufficient, by the side window itself. Changes in the capacitance occur if there is an obstacle present in a slot between the slot boundaries. A disadvantage of the known device is that the distance of the electrodes forming the slot boundaries is changed by moving the side windows so that the resulting change in capacitance has to be taken into account when generating the control signal.
A proximity anti-trap guard also reads from EP 1 154 110 A2. The way in which this known anti-trap guard works is based on a change in the capacitance prompted by an electrically conductive obstacle in an electric field generated between two electrical conductors. One of the two conductors, the sensor electrode, is integrated in a weatherseal sealing the closure element and receives a predefined electrical charge. It is in this way that a measuring capacitance can be determined between the sensor electrode and the other conductor, the base electrode which is grounded, for instance. This change in the measuring capacitance by an obstacle is determined by means of a control unit which provides a control signal for a drive powering the closure element. To also detect non-conducting materials, such as for example, wood or plastics causing no or only a very slight change in capacitance, this capacitive anti-trap guard features a soft-spot. This soft-spot is achieved by the portion of the weatherseal mounting the sensor electrode being configured deformable. In this way, contact of the obstacle with the weatherseal prompts a change in the position of the sensor electrode in relation to the base electrode, resulting in a change in the capacitance.
The last-mentioned document discloses in addition methods of monitoring proper functioning of sensor electrode and base electrode. A first method involves applying a DC signal to the open end of a sensor electrode configured in the form of a loop. A second method provides for applying to a sensor electrode featuring a free end a predefined signal which is reflected at the free end. From the differences in the transit time an open circuit in the sensor electrode, for example, can be detected. A third method involves comparing the capacitance between sensor electrode and base electrode to a predefined reference capacitance.
Described also in U.S. Pat. No. 6,389,752 B1 is a method for monitoring proper functioning of a tactile anti-trap guard. This known method involves circuiting a resistance between two electrical conductors, the resistance providing a constant impedance. The resistance reflects a signal pulse fed to the conductors, the transit time of which is detected by a control unit. Any deficiency in proper functioning of the anti-trap guard is detected by the difference in the transit time.
The disadvantage with such proximity devices is that the electric field generated by the sensor electrode is subject to disruptive effects which may detriment proper functioning and reliable operation. Thus, motion of the closure element is disruptive to the reference capacitance. Apart from this, malfunctioning may be caused by electromagnetic interference. Although electromagnetic compatibility (EMC) is achievable to a sufficient degree in principle by an electronic compensating circuit this makes the device unreasonably complicated.